U.S. patent number 7,386,097 [Application Number 11/257,765] was granted by the patent office on 2008-06-10 for analytical instrument with variable apertures for radiation beam.
This patent grant is currently assigned to Broker AXS, Inc.. Invention is credited to Arne Kasten, Gijsbertus J. Kerpershoek, Leendert J. Seijbel, Arjen B. Storm.
United States Patent |
7,386,097 |
Kerpershoek , et
al. |
June 10, 2008 |
Analytical instrument with variable apertures for radiation
beam
Abstract
An X-ray analysis device makes use of a variable aperture for
controlling the position and cross section of the X-ray beam. The
variable aperture is configured to allow changes in the cross
section and/or position of the beam by movement of one aperture
component in one direction. In one embodiment, the aperture medium
is a perforated disk that is rotated to expose different aperture
holes to the beam. In another embodiment, the aperture medium is a
perforated tape that is moved in a linear direction to expose
different aperture holes to the beam. The tape may be wound about
two axes to control its movement, or may be a continuous loop. A
cassette may also be used to house the tape.
Inventors: |
Kerpershoek; Gijsbertus J.
(Barendrecht, NL), Seijbel; Leendert J. (Rotterdam,
NL), Storm; Arjen B. (Den Haag, NL),
Kasten; Arne (Karlsruhe, DE) |
Assignee: |
Broker AXS, Inc. (Madison,
WI)
|
Family
ID: |
36201695 |
Appl.
No.: |
11/257,765 |
Filed: |
October 25, 2005 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20060108534 A1 |
May 25, 2006 |
|
Foreign Application Priority Data
|
|
|
|
|
Oct 28, 2004 [DE] |
|
|
10 2004 052 350 |
|
Current U.S.
Class: |
378/149;
378/148 |
Current CPC
Class: |
G01N
23/20008 (20130101) |
Current International
Class: |
G21K
1/02 (20060101) |
Field of
Search: |
;378/145-146,147,148-149,161,84,150 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
102 57 206 |
|
Jul 2004 |
|
DE |
|
08285798 |
|
Jan 1996 |
|
JP |
|
10142171 |
|
May 1998 |
|
JP |
|
WO01/44793 |
|
Jun 2001 |
|
WO |
|
Other References
Bruker AXS, "Structural Biology Solutions, X8 Proteum X8
Prospector", Bruker Advanced X-Ray Solutions, 2005, pp. 1-25,
Germany. cited by other.
|
Primary Examiner: Song; Hoon
Attorney, Agent or Firm: Law Offices of Paul E. Kudirka
Claims
What is claimed is:
1. An X-ray diffraction apparatus comprising: an X-ray generator
for generating an X-ray beam that propagates in a beam direction to
illuminate a sample in order to make an X-ray diffraction
measurement; a movable component having a plurality of apertures
with identical sizes and shapes, but located at different positions
along a first direction perpedicular to the beam direction, the
component being positioned between the generator and the sample so
that the X-ray beam illuminates the sample through a single first
aperture; and a mechanism for moving the component in a second
direction different from the first direction to cause the X-ray
beam to illuminate the sample through another single aperture with
a position that is shifted in the first and second directions from
the position of the first aperture.
2. An apparatus according to claim 1 wherein the movable component
comprises an aperture medium that is substantially opaque to the
X-ray beam and within which a plurality of holes are located.
3. An apparatus according to claim 2 wherein the aperture medium
comprises a disk that is moved rotationally.
4. An apparatus according to claim 3 wherein rotation of the disk
by a predetermined angular distance moves a first one of the holes
out of alignment with the beam and a second one of the holes into
alignment with the X-ray beam.
5. An apparatus according to claim 4 wherein the second hole has a
different radial position than the first hole relative to an axis
about which the disk is rotated.
6. An apparatus according to claim 5 wherein at least some of the
holes in the disk are aligned in a row that, from one end of the
row to an opposite end, is characterized by the incremental
increase in radial position of the holes in the row relative to an
axis about which the disk is rotated.
7. An apparatus according to claim 6 wherein all of the holes of
the row have the same cross-sectional size and shape.
8. An apparatus according to claim 6 wherein the row is a first row
and wherein the holes of the disk are organized into a plurality of
rows located in different radial segments around the disk.
9. An apparatus according to claim 8 wherein the holes of a given
row all have the same cross-sectional size as each other, and a
cross-sectional size different from that of the holes in the other
rows.
10. An apparatus according to claim 4 wherein the disk rotation is
driven by a motor.
11. An apparatus according to claim 10 further comprising a
position sensor that detects indicia of the disk to determine the
rotational position of the disk.
12. An apparatus according to claim 11 wherein the indicia comprise
the holes of the disk.
13. An apparatus according to claim 11 further comprising a
controller that controls the position of the disk so as to align a
chosen one of the holes with the beam.
14. An apparatus according to claim 2 wherein the aperture medium
comprises a tape that is moved in a linear direction in the
vicinity of the beam.
15. An apparatus according to claim 14 wherein movement of the tape
by a predetermined distance moves a first one of the holes out of
alignment with the beam and a second one of the holes into
alignment with the beam.
16. An apparatus according to claim 15 wherein the second hole has
a different lateral position than the first hole relative to a
direction perpendicular to the direction along which the tape is
moved in the vicinity of the beam.
17. An apparatus according to claim 16 wherein at least some of the
holes in the tape are aligned in a row that, from one end of the
row to an opposite end, is characterized by the incremental
increase in lateral position of the holes in the row relative to a
direction perpendicular to the direction along which the tape is
moved in the vicinity of the X-ray beam.
18. An apparatus according to claim 17 wherein all of the holes of
the row have the same cross-sectional size and shape.
19. An apparatus according to claim 17 wherein the row is a first
row and wherein the holes of the tape are organized into a
plurality of rows located in different linear segments along the
length of the tape.
20. An apparatus according to claim 19 wherein the holes of a given
row all have the same cross-sectional size as each other, and a
cross-sectional size different from that of the holes in the other
rows.
21. An apparatus according to claim 15 wherein the tape movement is
driven by a motor.
22. An apparatus according to claim 15 wherein the tape is wound
onto take-up reels that are rotated to move the holes on the tape
relative to the X-ray beam.
23. An apparatus according to claim 22 further comprising a
position sensor that detects indicia of the tape to determine the
position of the tape relative to the X-ray beam.
24. An apparatus according to claim 23 wherein the indicia comprise
the holes of the tape.
25. An apparatus according to claim 23 further comprising a motor
that drives one of the take-up reels and a controller that controls
the motor to position the tape so as to align a chosen one of the
holes with the X-ray beam.
26. An apparatus according to claim 22 further comprising a
cassette within which the tape and take-up reels are housed, the
cassette having apertures that allow the X-ray beam to enter a
first side of the cassette, encounter the tape, and exit a second
side of the cassette.
27. An apparatus according to claim 15 wherein the tape forms a
closed loop.
28. An apparatus according to claim 27 wherein the tape forms a
Mobius loop.
29. An apparatus according to claim 2 wherein the aperture medium
has a minimum of thirty-six holes.
30. An apparatus according to claim 1 further comprising at least
one fixed aperture positioned between the generator and the sample
along with the movable component.
31. A method for operating an X-ray diffraction apparatus, the
method comprising: generating an X-ray beam that propagates in a
beam direction to illuminate a sample in order to make an X-ray
diffraction measurement; positioning a movable component between
the generator and the sample, the movable component having a
plurality of apertures with identical sizes and shapes, but located
at different positions along a first direction perpendicular to the
beam direction, so that the X-ray beam illuminates the sample
through a single first aperture; and moving the component in a
second direction different from the first direction to cause the
X-ray beam to illuminate the sample through another single aperture
with a position that is shifted in the first and second directions
from the position of the first aperture.
Description
FIELD OF THE INVENTION
This invention relates, generally to the use of variable apertures
for high energy radiation in analysis equipment and, more
specifically, variable apertures used for limiting the size and
shape of an X-ray beam in a single crystal X-ray
diffractometer.
BACKGROUND OF THE INVENTION
An X-ray optical device with a variable aperture is discussed in US
Patent Application Publication No. US 2004/0170250 A1. In this
device, the variable aperture consists of a fixed portion and a
movable portion that moves relatively to the fixed portion to
change the size of an opening formed between the portions. These
portions together are capable of shaping an X-ray beam passed
through the aperture to a rectangular cross-section. As the movable
portion is moved, the cross-section and position of the X-ray beam
output from the device are changed. Notably, it is not possible to
change the beam position while maintaining a consistent
cross-section, or to change the beam cross-section while
maintaining the position, unless both portions of the aperture are
exchanged for aperture portions of different configurations.
A known X-ray diffractometer (the "X8 Proteum" produced by Bruker
AXS, Madison, Wis.) uses a beam aperture that is mounted in a
holder so that it is positioned perpendicularly to an incident
X-ray beam. The aperture limits the cross-section of the beam, and
its position can be adjusted in two orthogonal directions prior to
its being fixed in place. This allows a user to fine-tune the
position of the transmitted beam as part of an initial setup.
Different beam cross-sections may also be selected by choosing from
apertures with different shapes. However, to accomplish any of
these changes requires time-consuming adjustments of the aperture,
either by repositioning in two independent directions or by the
manual exchange of the aperture.
SUMMARY OF THE INVENTION
In accordance with the present invention, an analysis apparatus
that uses a radiation beam that illuminates a sample has at least
one beam-limiting aperture apparatus that provides an aperture
through which the beam must pass to reach the sample. The aperture
apparatus is adjustable to vary at least one of the cross section
and the position of the aperture by movement of a single movable
component of the aperture apparatus in one direction. The movable
component may be an aperture medium that is substantially opaque to
the radiation beam and within which a plurality of holes are
located. Movement of the aperture medium relative to the radiation
beam changes which one of a plurality of different holes is located
in the beam path. The different holes may be at different positions
relative to the beam and may have different sizes or shapes.
In one embodiment of the invention, the aperture apparatus
comprises a disk that can be rotated about an axis. The disk uses a
material that is not transparent to the beam radiation and that has
a plurality of holes that are positioned within a radial range from
r.sub.min to r.sub.max relative to a center of the disk. The disk
may be positioned with a primary surface perpendicular to the beam,
and the holes are arranged such that rotation of the disk by a
predetermined angular increment changes the hole that is in the
path of the beam. The disk may also be arranged such that, in each
rotational position, taking into account the maximum beam
cross-section at that position, radiation passes through only one
of said holes. Some of the holes may have an identical
cross-section, but different radial positions within the range
r.sub.min to r.sub.max, such that the transition from one hole to
another one results in a radial shift of the effective aperture. As
each of the holes passes through the range of the beam, rotation of
the disk also results in a change in the angular position of that
hole. Thus, the apparatus allows a shift of the aperture in two
dimensions by only one movement, namely, the rotation of the
disk.
In another embodiment of the invention, the aperture apparatus
comprises a tape of width b.sub.s that resides in a plane that
intersects the beam direction, and that can be shifted across the
beam path in a first direction. As with the disk, the tape is made
from material that is not transparent to the beam radiation, and
the tape has a plurality of holes located at different positions
along a second direction perpendicular to the first direction.
Holes at different positions along this second direction, or width,
of the tape therefore intersect the beam in different regions of
the beam cross section. These positions may span a given range,
such as between b.sub.min and b.sub.max, and holes at each of these
positions produce an output beam at a different positional shift
along the second direction. Thus, by moving the tape along the
first direction by a given increment, holes at different positions
along the second direction may be positioned to intersect the beam,
and to thereby produce an output beam with a different positional
shift. As with the disk embodiment, the holes in the tape may be
arranged relative to one another, taking into account the maximum
beam cross-section at the tape position, so that radiation passes
through only one of the holes at a time in a direction that can
reach a sample being analyzed. The holes may have an identical
cross-section but different width positions between b.sub.min and
b.sub.max, such that, as mentioned above, the transition from one
hole to the next can be used to effect a positional shift in the
effective aperture in the second direction. While the beam is
illuminating a given hole, a slight shift of the tape in the first
direction results in the output from a hole in the path of the beam
to be shifted in the first direction, thereby allowing a positional
shift of the effective aperture in the first direction. Thus,
shifting of the aperture in two dimensions may be accomplished by
changing only one parameter, namely, the movement of the tape in
the first direction.
In the tape embodiment, the tape may be controlled by winding two
opposite ends of the tape on take-up reels positioned to opposite
sides of the beam. The tape is held under tension between the
reels, and its position may be shifted by rotating one of the
reels, for example, by using a motor attached by a shaft to the
reel. The plane of the tape which intersects the beam path, may be
kept in a desired orientation relative to the beam by using guides
through which the tape must pass and which keep the relevant
portion of the tape at the desired orientation. The guides might
also include a guide that has a closed loop shape in a plane
perpendicular to the beam such that the beam passes through the
closed loop of the guide. In such a case, the guide may also serve
as a maximum aperture for the beam while a hole that is within the
cross section of the beam is located within the closed loop shape
of the guide.
The tape may also be mounted in a cassette, similar to an audio or
video cassette, which may be exchangeable in the analysis
apparatus. The cassette housing is configured such that it does not
interfere with the beam, which passes through it to encounter the
tape. For example, the cassette itself may have apertures that
align with the relevant portion of the tape along the path of the
beam. The use of a cassette allows for a simple exchange of
different tapes having different hole configurations. In one
variation of this embodiment, the tape itself is configured as a
closed loop that may be repeated drawn through the path of the beam
in the same direction. This allows different positions on the tape
to be repeatedly accessed while moving the tape loop in only one
direction. The loop may also be in the shape of a Mobius band, such
that, after one revolution, upper and lower parts of the tape are
reversed, providing twice as many (symmetric) positions of the
holes.
It may be desirable to have a total number of holes in the disk or
the tape be greater than 12, or even greater than 36. When using
the disk, the number of holes may be limited by the surface area of
the disk and the radial positioning of the holes. When using a
tape, there is no such limitation on the number of holes, which may
number much higher, e.g., in the hundreds. The larger the number of
holes, the more precisely the beam can be shifted in the second
dimension and/or the number of different hole cross-sections can be
increased.
In one example, the holes may be aligned in rows, each with a
particular number of holes. For example, there may be j rows, each
with i holes. Each of the rows may have holes that all have an
identical cross-section, but that are all positioned slightly
differently. Thus, a desired cross section may be shifted in two
directions by changing the position of the disk or tape along the
range of a given row.
It may be desirable to locate the holes of a row, in a dimension
perpendicular to the movement of the medium, so that the hole
position from one hole to the next changes incrementally in a
constant direction. On a disk, this would be effected by locating
each hole in a row slightly further from the center of the disk
than an adjacent hole. On a tape, each hole of a given row could be
offset incrementally further along the second direction than a
previous hole. This arrangement would allow the position of a beam
to be shifted incrementally in a quasi-continuous manner by
shifting from one hole to the next of a given row. Such shifts of
the beam position may be particularly useful for illuminating or
obscuring different parts of focusing optics, which may influence
the magnification ratio of the optics.
It may also be desirable to arrange the holes such that the number
of holes on the disk or tape is maximized with the constraint that,
in each position of the disk or tape, the beam does not pass
through two neighboring holes simultaneously. Similarly, the holes
may be configured such that a given hole can be moved within the
beam width, perhaps across the entire beam cross-section, without
radiation passing through a neighboring hole simultaneously. This
"closest packing" of the holes allows the most compact arrangement
of holes for a given amount of hole sizes and/or positions.
In the analysis system, it may be desirable to provide at least one
fixed aperture in the beam path, in particular directly before
and/or behind the disk or tape. The cross-section of this aperture
may be matched to the largest available beam cross-section and/or
to the cross-section of the largest hole. The fixed aperture may
also be sized to match the extent of the range of the holes in the
direction perpendicular to the movement direction of the aperture
medium, such as the range r.sub.max-r.sub.min for the disk, or
b.sub.max-b.sub.min for the tape. Altogether an aperture in the
form of a broad gap is preferred. This fixed aperture can also be
exchangeable.
In one implementation of the invention, the variable aperture is
positioned in the analysis system in the beam path between source
and focusing optics. In one variation, the variable aperture is
positioned close to the focusing optics, and possibly directly
before the focusing optics. With this arrangement the beam
divergence can be adjusted optimally. The variable aperture may
also be positioned in the beam path between the focusing optics and
the sample, possibly close to the focusing optics, or even directly
behind the focusing optics. The variable aperture may also be
positioned in the beam path directly before the sample to enable an
optimal illumination and/or masking.
The variable aperture apparatus may also be driven by a motor, and
its movement may be controlled by a controller. The motor may be,
for example, a stepper motor. This would allow a particularly
precise adjustment of the desired positions. The positions may also
be pre-set and/or stored for renewed alignment. To this end, the
system may also include a sensor for determining the position of
the variable aperture. An optical sensor in combination with an
encoder may be attached to the variable aperture to detect the
position of the aperture medium, or the sensor may detect position
based on detection of the position of those holes that are not
currently in the path of the beam. The aperture apparatus may also
be configured to allow the easy exchange of aperture media.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and further advantages of the invention may be better
understood by referring to the following description in conjunction
with the accompanying drawings in which:
FIG. 1 is a schematic view of an X-ray beam analysis system that
uses a variable aperture according to the invention;
FIG. 2a is a schematic, perspective view of a variable aperture
apparatus having a disk-shaped aperture medium;
FIG. 2b is a schematic, front view of a disk such as that used in
the apparatus of FIG. 2a;
FIG. 3 is a schematic view of a variable aperture medium in the
form of a tape;
FIG. 4 is a schematic, perspective view of a variable aperture
apparatus that may use the tape shown in FIG. 3;
FIG. 5 is a schematic, perspective view of a cassette mechanism
that may be used with the variable aperture apparatus of FIG. 4;
and
FIG. 6 is a schematic, perspective view of a variable aperture
medium in the form of a tape shaped like a Mobius band.
DETAILED DESCRIPTION
Shown schematically in FIG. 1 is an analysis device 10 according to
the invention which comprises a single crystal diffractometer. A
sample 14, in particular a single crystal, is illuminated by an
X-ray beam 16 via focusing X-ray optics 13 which, in this
embodiment, may be an X-ray mirror and, more particularly, a
multilayer mirror such as a Gobel mirror or a Montel mirror. The
focus of the beam 16 may be chosen to be on the sample or,
alternatively, on detector 15 located behind the sample 14. The
optimal position of the beam focus depends on the required
experimental conditions. X-rays 17 scattered by the sample 14 are
detected by the detector 15, which may be an area detector. A
diffraction pattern of the sample is formed on the detector
surface, and is detected and resolved by the detector and evaluated
in a subsequent processing step.
Positioned at several locations along the path of the X-ray beam 16
are apertures or masks 12a-12d through which the beam 16 must pass.
A first aperture 12a is located immediately before the X-ray optics
13 along the beam path, a second aperture 12b is located after the
X-ray optics 13 but in close proximity, a third aperture 12c is
located further along the beam path toward the sample, and a fourth
aperture 12d is located even further along the path, in close
proximity to the sample 14. In this embodiment, apertures 12b and
12c are attached to each other and combine to form an expanding
conical pipe, although this configuration is not necessary to the
invention. Each of the apertures 12a-12d is either fixed or
exchangeable, and at least one of the apertures is a variable
aperture.
FIGS. 2A and 2B show components of a first embodiment of a variable
aperture according to the invention that may be used in a system
like that of FIG. 1. In this embodiment, the aperture is a
perforated disk 12 with radius r.sub.s. The disk 12 comprises a
material that is not transparent for the X-ray wavelengths used,
typically some type of metal. The particular material, and its
thickness, may be selected based on known data for common materials
for the particular X-ray radiation used, which may be, for
instance, Cu-K.sub.alpha radiation. Along the face of the disk are
a plurality of rows 21, 22, 23, 24, 25, 26 of holes, each of which
is arranged so that it follows a path of changing radius relative
to the center of the disk. In the example shown in FIGS. 2A and 2B,
the total number "j" of rows is equal to 6, and each row is at a
radius r.sub.min at one end of the row, and at a radius r.sub.max
at the other end of the row. In the example shown, the total number
of holes "i" for each row is equal to eleven, and each row has the
same average radial distance to the center of the disk. In
addition, the rows have an equal relative angular spacing about the
disk. However, those skilled in the art will recognize that the
specific configuration of holes on the disk may be varied as
appropriate for a particular application.
As shown in FIG. 2A, the surface of disk 12 is oriented roughly
perpendicularly to the beam 16, and the positions and distances
between the holes are such that, when the angular position of a
hole is aligned with the angular position of the beam only one hole
is illuminated. Alternatively, the distances between the holes are
slightly larger, such that each hole can be somewhat moved
perpendicular to the beam without illuminating a neighboring hole.
The disk 12 may be rotated using a motor 27, which is controlled by
controller 29 of the analysis device 10. The motor 27 drives a disk
shaft 28, which brings the holes into the path of the beam. Each
hole can be aligned around the beam center in the angular
direction, and the angular position of the transmitted beam can be
varied in the range of the beam cross-section by slightly varying
the angular position of the hole.
While holes of the same row are being sequentially exposed to the
beam, rotation of the disk causes a shift in radial position of the
transmitted beam due to the transition between neighboring holes.
Each of the holes of a given row has a slightly different radial
position so, with each hole, a different portion of the beam is
transmitted through the disk, and each of these beam portions is
also at a different radial position. The effective result is,
therefore, that changing from one hole to the next changes the
radial position of the beam exiting the aperture. In the example
shown, there are six rows of holes, each row having holes with a
cross-section that differs from the holes of the other rows. Within
a given row, the holes all have the same cross section, but differ
from one another by their radial position. These characteristics,
of course, are specific to this example, and those skilled in the
art will understand that the arrangement and cross section of the
holes may take on any of a number of different configurations.
Similarly, although the cross-sectional shape of the holes in the
disk 12 is circular, the holes may just as well be square or
elliptical, as may be desired. Moreover, the disk may be tilted
relative to a plane perpendicular to the beam direction, thereby
changing the effective cross-sectional shape of the holes (e.g.,
circular holes would effectively become elliptical holes). A
variation such as this may also be applied, if desired.
A positional marking may also be delineated on the disk, and used
for monitoring the disk position. In the example shown in FIGS. 2A
and 2B, the marking 31 consists of a series of perforations in the
disk at a radial distance further from the center of the disk than
the rows of holes. This marking may be read by a device such as an
optical scanner, represented in the figure by light source 32 and
detector 33. The detection of the code with the optical scanner may
be used to identify the angular position of the disk 12 and to
generate a signal to the controller 29 indicative thereof. The
controller may thereby use that information to control the angular
position of the disk. As an example, a disk position read from the
code may be used to establish a zero-position for the stepper motor
27.
Depending on the desired beam profiles and beam positions,
different disks 12 may be attached to the shaft 28, the different
disks having different arrangements of holes. If desired, different
constraints may be maintained from one disk to the next, such as
requiring that, during measurement, only one hole is illuminated by
beam 16 at a given time. The various configurations, however, may
be within the control of a system user.
As shown in FIG. 2B, there may also be a limiting aperture 34
directly in front of or behind the disk 12 that limits the cross
section of the beam that is actually encountered by the disk. In
the example shown in the figure, the limiting aperture 34 is sized
to cover only the range of r.sub.max-r.sub.min, which is the range
over which the radial position of the holes on the disk varies.
This limiting aperture 34 remains stationary as the disk rotates
and, optionally, may be exchangeable with other apertures. In this
example, the limiting aperture 34 has a width that corresponds to,
or is slightly larger than, the maximum width of a hole adjacent to
it. Different limiting apertures may also be used as may be
appropriate with a particular embodiment.
The disk-shaped aperture medium of FIGS. 2A and 2B provides a basis
for variable apertures that may be used in the example of FIG. 1 as
one or more of the apertures 12a-12d. Another type of variable
aperture according to the present invention uses an aperture medium
that may be moved linearly, rather than rotationally. FIG. 3 is a
schematic depiction showing a perforated tape 36 that is a part of
a second embodiment of the present invention. The tape 36 has rows
of holes along its length, and it may be shifted in the x-direction
indicated in the figure. The rows 38, 40, 42, 44 and 46 of holes
vary in position along the y-direction, which is perpendicular to
the x-direction. In this example, each row has holes that are of a
constant cross section, and that vary in position incrementally
relative to the y-direction. As the tape is moved in the
x-direction, the y-direction may be viewed as being along the width
of the tape. If the tape is illuminated with an X-ray beam, and
only one of the holes is in the path of the beam at a given time,
the beam exiting the other side of the tape will have a position
along the y-direction that depends on which hole is in the path of
the beam. Shifting the tape to move one hole of a row out of the
beam path, and to move another hole of that row into the beam path,
has the effect of shifting the position of the output X-ray beam in
the y-direction. Shifting a different row into the path of the beam
has the effect of changing the cross-sectional size of the output
beam. In the example of FIG. 3, each of the rows has j=10 holes in
different positions along that width from a lower limit b.sub.min
to an upper limit b.sub.max.
For the tape embodiment, the number of rows can be significantly
larger than the number of rows used for the disk embodiment of
FIGS. 2A and 2B. Otherwise, the tape embodiment of FIG. 3 operates
in a manner analogous to the embodiment of FIGS. 2A and 2B, where
the shift of the FIG. 3 tape in the x-direction corresponds to a
rotation of the FIGS. 2A and 2B disk. Similarly, the distance in
the y-direction spanned by the holes in the tape 36 corresponds to
the distance spanned by the holes along a radial dimension of the
disk of FIGS. 2A and 2B. The FIG. 3 embodiment may also use at
least one limiting aperture 48 that is similar to the aperture 34
shown in FIG. 2B. Like the disks, tapes can also be exchanged if
necessary and can be mounted at any of the aperture positions
12a-12d shown in FIG. 1. Depending on the manner of tape transport,
the flexibility of the material and thickness of the tape may vary.
In general, the desired tape thickness depends on the type of
radiation used and by the tape material itself.
FIG. 4 shows an embodiment of a tape transport system for use with
an aperture tape like that of FIG. 3. In this embodiment, the tape
36 is wound under tension about two axes, in this example, making
use of take-up reels 50a and 50b. The rotation of the take-up reels
may be driven by a motor 52 that may be connected to one of the
reels via a drive shaft. The two take-up reels 50a, 50b, together
with fixed aperture guide 53, keep the tensioned tape straight and
perpendicular to the beam 16 in the area where the beam 16
encounters the tape. The beam passes through limiting aperture 48,
as discussed above. A tape position sensor 54 may make use of a
light source such as LED 56 and a detector 58. By illumination of a
set of markings 60, which may comprise a line of holes in the tape
36, and detection of the light passing through the holes, the tape
position may be monitored and used by a controller to control the
motor 52.
FIG. 5 shows the tape system of FIG. 4 mounted in a cassette 62
that houses the system components. The tape 36 may be wound back
and forth between the take-up reels, which are mounted inside the
cassette. In this example, the cassette 62 also houses the detector
58 used by the tape position sensor, although those skilled in the
art will understand that the detector may also be mounted outside
of the cassette, for example, in an adjacent analysis device. The
cassette housing itself contains two separate apertures 64, 66,
which allow the beam to pass into the cassette to where it
encounters the tape 36, and then out the other side. Mounting
brackets 68 may be used to accurately position the cassette in the
system.
A variation of the reel-to-reel tape embodiment of FIGS. 4 and 5 is
shown FIG. 6. In this embodiment, a tape 70, rather than being
linear and rolled onto take-up reels, may be implemented as a
closed loop. In this example, the movement of the tape may be
driven by a motor 72 and guided by several drums 74. In the area
where the beam meets the tape, the tape will be flat and
perpendicular to the beam. However, in this example, the tape is in
the shape of a Mobius band. Thus, after one revolution the
positions of the front and back side of the tape, and the upper and
lower edge of the tape, are exchanged. Thus, if the holes in the
tape are biased to one side along the width of the tape, one
revolution of the tape reverses those positions along the tape
width. This effectively doubles the number of positions for holes
along the tape width, and provides the system with greater
versatility for a given length of tape. Those skilled in the art
will recognize that the other characteristics discussed for the
previous examples apply in a corresponding way to this embodiment
of a variable aperture.
While the invention has been shown and described with reference to
the embodiments thereof, those skilled in the art will recognize
that various changes in form and detail may be made herein without
departing from the spirit and scope of the invention as defined by
the appended claims.
* * * * *